How Many DPO Am I

OK all you experts out there. I feel really good about this month. Depending on what app I check, I either ovulated on CD 17 or CD 19, though I suspect it was 18. I know it doesn’t make a huge difference but I feel like I might be pregnant and the TWW is killing me! lol I got a blindingly white BFN today but I think it’s just too early. Can you please indulge my lunacy and help me pinpoint my O date? Thanks!